The present invention relates to a low-distortion cascode circuit for high-frequency communications, and relates more particularly to a low-distortion cascode circuit corresponding to a frequency converter or an amplifier of excellent low-distortion characteristics.
As a frequency converter (mixer) for a communication unit, a cascode circuit using a dual-gate field effect transistor (hereinafter to be referred to as a FET) has been widely known. An example of the conventional dual-gate FET is disclosed in the "GaAs Dual-Gate MESFETS's IEEE, Trans. Electron Devices, vol. ED-25, pp. 580-586", issued in June 1978.
The above-described prior-art technique is characterized in that the dimensions of two FETs are set the same. The prior-art dual-gate FET can be considered to be the cascode configuration of the FETs having almost the same electric characteristics.
The outline of a dual-gate FET which is operated as a frequency converter will be explained with reference to FIG. 6. Equivalent circuits of the dual-gate FET are shown as a cascode connection of two FETs 1 and 2. When the dual-gate FET is operated as a down-converter (mixer), an RF signal is applied to the gate of the lower-stage FET 1 from a terminal 5. A local signal is applied to the gate of the upper-stage FET 2 from a terminal 4.
Functions of the FETs 1 and 2 will be explained below. The FET 1 functions as a voltage control type resistor which is modulated by an RF signal applied to the gate. The FET 2 has two functions. One is the function of a source follower for applying a local signal to the resistor. Since a local signal is applied to the resistor that has been modulated by the RF signal, a component of a product of both signals occurs in a current Id that flows through the resistor, so that a mixing function is achieved. The other function of the FET 2 is the function of an amplifier of a gate ground type. The current Id that flows through the resistor is outputted from the drain.